External pipe welding apparatus

ABSTRACT

The invention relates to the field of automated welding, and more particularly, to an automated, gas metal arc welding apparatus for externally welding abutting sections of pipe. The apparatus has a motorized main carriage for travelling circumferentially about one of the pipe sections to be welded. The apparatus is provided with a welding torch which is connected to the main carriage. The welding torch is movable relative to the main carriage by the actuation of a multi-axis drive mechanism. The multi-axis drive mechanism has a horizontal transport mechanism for linearly translating the torch transversely of the weld seam, a vertical transport mechanism for vertically displacing the torch transversely of the weld seam and an angular transport mechanism for pivotally moving the torch transversely of the weld seam. A controller is provided for actuating the various transport mechanisms individually or simultaneously if desired.

FIELD OF THE INVENTION

[0001] The invention relates to the field of automated welding, and moreparticularly, to an automated, gas metal arc welding apparatus forexternally welding abutting sections of pipe.

BACKGROUND OF THE INVENTION

[0002] Pipelines such as those used for the conveyance of oil or waterare constructed by welding sections of pipe together. Often, the weldingactivities will be carried out by automated welding systems at theinstallation site of the pipeline. In some instances, automated weldingsystems capable of being moved along the pipeline right-of-way may beused.

[0003] Such mobile automated welding systems may comprise a weldingcarriage and a welding assembly, which includes a torch suitable for GasMetal Arc Welding (GMAW). The welding carriage is adapted to carry thewelding assembly around an annular track or guide mounted externallyabout one of the pipe sections. Typically, the welding carriage willhave a motor for driving the carriage on the guide.

[0004] In the field, a variety of welding situations may presentthemselves. For example, the pipe sections to be welded may havelarge-angled bevels resulting in broad weld grooves. Conversely, otherpipes having deep, thick sections will require that the weld beperformed along a narrow weld seam. A flexible welding system is soughtthat is adaptable to such diverse conditions such that the weldingoperation can be effect3ed with a single, mobile, apparatus. Inaddition, the welding system should be flexible enough to enable asubstantially constant heat input to the weld.

SUMMARY OF THE INVENTION

[0005] In an aspect of the invention there is an apparatus for weldingabutting pipe sections along a weld seam, the apparatus comprising: amotorized main carriage for travelling circumferentially about one ofthe pipe sections to be welded; a welding torch connected to the maincarriage, the welding torch being movable relative to the main carriage;a first motorized transport mechanism connected to the main carriage forvertically displacing the welding torch relative to the weld seam; and asecond motorized transport mechanism connected to the main carriage forpivotally moving the welding torch transversely of the weld seam.

[0006] In an additional feature of the invention the apparatus includesa controller for actuating the second transport mechanism so as topivotally oscillate the welding torch transverse of the weld seam. Inyet another additional feature, the controller actuates the first andthe second transport mechanisms simultaneously so as to change thevertical position of a pivoting point of the torch in accordance with apre-determined trajectory. In still another additional feature, thecontroller receives feedback as to a welding current, a welding voltageand actuates the first transport mechanism so as to effect apre-determined minimum heat input into the weld seam. In a furtheradditional feature, the controller prioritizes the function of effectingsaid pre-determined minimum heat input over the function of followingsaid pre-determined trajectory for the pivoting point of the torch.

[0007] In another additional feature, the apparatus further comprises athird motorized transport mechanism connected to the main carriage forlinearly translating the welding torch transversely of the weld seam. Inyet another additional feature, the apparatus includes a controller foractivating the third transport mechanism so as to linearly oscillate thewelding torch transverse of the weld seam. In still another additionalfeature, the controller is operable to actuate the first, the second andthe third transport mechanisms simultaneously in order to effect apre-determined trajectory and angular orientation of the torch relativeto one of the vertical or transverse directions.

[0008] In another aspect of the invention, there is an apparatus forwelding abutting pipe sections along a weld seam, the apparatuscomprising: a motorized main carriage for travelling circumferentiallyabout one of the pipe sections to be welded; a welding torch connectedto the main carriage; the torch being movable relative to the maincarriage; a first motorized transport mechanism connected to the maincarriage for linearly translating the torch transversely of the weldseam; and a second motorized transport mechanism connected to the maincarriage for pivotally moving the torch transversely of the weld seam.

[0009] In an additional feature of the invention, the apparatus includesa controller for actuating the second transport mechanism so as tooscillate the torch about a pivot point, transverse of the weld seam;and for actuating the first transport mechanism so as to linearlytranslate the pivot point transverse of the weld seam. In yet anotheradditional feature, the first transport mechanism includes: a carriageconnected to the welding torch, a motor for providing a driving force toeffect linear translation of the welding torch; and a frame forsupporting the motor. The frame is fixed to the main carriage. Thecarriage is slidably mounted to the frame and translatable relative tothe main carriage in a direction transverse of the weld seam. In stillanother additional feature, the carriage has guide means fixed thereto.The guide means are slidably mounted to the frame. In a furtheradditional feature the first transport assembly includes driving meansinterconnecting the motor and the carriage. In yet another additionalfeature the driving means include: a cog rotatably mounted to the motorfor transmitting the driving force of the motor; a cog pulley rotatablyconnected to the cog and mounted to extend through the frame, the cogpulley including a threaded shaft rotatable about the frame; and a nutfixed to the carriage for threadingly engaging the shaft. The cog pulleyis adapted to receive the driving force of the motor and to rotate theshaft. The nut is adapted is to travel along the shaft when the motor isactuated to thereby urge the carriage to translate transversely of theweld seam.

[0010] In another additional feature, the second transport assemblyincludes: a frame connected to the main carriage; a mount for carryingthe welding torch; a motor for providing a driving force to effectpivotal motion of the welding torch, the motor being fixed to the frame;an eccentric cam connected to the motor; and a cam follower disposedintermediate the cam and the mount and pivotally attached to the frame.The cam follower is adapted to receive the driving force from the motorsuch that when the motor is actuated the cam follower is urged to rotateabout the frame thereby causing the welding head to move pivotally in adirection transverse of the weld seam.

[0011] In yet another additional feature the second transport assemblyincludes: a frame connected to the main carriage; a mount for carryingthe welding torch; a motor for providing a driving force to effectpivotal motion of the welding torch, the motor being fixed to the frame;an eccentric cam connected to the motor; and a cam follower disposedintermediate the cam and the mount and pivotally attached to the frame.The cam follower is adapted to receive the driving force from the motorsuch that when the motor is actuated the cam follower is urged to rotateabout the frame thereby causing the welding head to move pivotally in adirection transverse of the weld seam.

[0012] In still another additional feature the second transportmechanism includes: a frame connected to the main carriage; a mount forcarrying the welding torch; a motor for providing a driving force toeffect pivotal motion of the welding torch, the motor being fixed to theframe; and a gear train connected to the motor. The mount is pivotallyattached to the frame. The gear train is adapted to transmit the drivingforce from the motor to the mount to cause the welding head to movepivotally transverse of the weld seam.

[0013] In a yet another additional feature, the apparatus furthercomprises a third motorized transport mechanism connected to the maincarriage for vertically displacing the welding torch relative to theweld seam. In a further additional feature, the controller is operativeto actuate the first, second and third transport mechanismssimultaneously in order to effect a pre-determined trajectory andangular orientation of the torch relative to one the vertical ortransverse directions. In still another additional feature thecontroller receives feedback as to a welding current and welding voltageand actuates the third transport mechanism so as to effect apre-determined minimum heat input into the weld seam. In yet anotheradditional feature the controller prioritizes the function of effectingsaid pre-determined minimum heat input over the function of followingsaid predetermined trajectory for the pivoting point of the torch. Instill another additional feature the third transport mechanism includes:a carriage connected to the welding torch, a motor for providing thedriving force to effect the vertical displacement of the welding torch,and a frame for supporting the motor, the frame being connected to themain carriage. The carriage is slidably mounted to the frame andvertically translatable relative to the main carriage in a directionperpendicular to the pipe sections to be welded. In yet anotheradditional feature the carriage has guide means fixed thereto, the guidemeans being slidably mounted to the frame. In still another additionalfeature, the third transport assembly includes driving meansinterconnecting the motor and the carriage. In a further additionalfeature, the driving means include: a cog rotatably mounted to the motorfor transmitting the driving force of the motor; and a cog pulleyrotatably connected to the cog and mounted to extend through the frame.The cog pulley includes a threaded shaft rotatable about the frame. Thecarriage has a threaded bore. The bore is adapted to receive thethreaded shaft. The co pulley is adapted to receive the driving force ofthe motor and to rotate the shaft within the bore. The carriage isadapted to travel vertically along the shaft when the motor is actuated.

[0014] In another additional feature, the first transport mechanism hasa motor-driven carriage that is translatable relative to the maincarriage in a direction transverse of the weld seam. The frame of thethird transport assembly is fixed to the carriage of the first transportassembly. In yet another additional feature, the second transportassembly has a frame and a mount connected thereto to support thewelding torch. The frame of the second transport assembly is pivotallyconnected to the carriage of the third transport assembly.

[0015] In another additional feature, the frame of the third transportassembly is fixed to the main carriage; and the first transportmechanism has a motor-driven carriage that is translatable relative tothe main carriage in a direction transverse of the weld seam. Thecarriage of the first transport assembly is fixed to the frame of thethird transport assembly. In still another additional feature, thesecond transport assembly has a frame and a mount connected thereto tosupport the welding torch. The frame of the second transport assembly ispivotally connected to the carriage of the first transport assembly.

[0016] In yet another additional feature of the invention, the apparatusfurther comprises a retracting mechanism for rotating the welding torchaway from the weld seam. The retracting mechanism includes a clamp forsecuring the welding head in position.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] For a better understanding of the present invention and to showmore clearly how it may be carried into effect, reference is now made,by way of example and not of limitation, to the accompanying drawings inwhich:

[0018]FIG. 1 is a perspective view of a pipe welding apparatus accordingto a first embodiment of the invention, showing a single weldingassembly supported on a main welding carriage;

[0019]FIG. 2 is an elevational view of the pipe welding apparatus ofFIG. 1 taken in the direction of arrow “2”, showing a typicalinstallation of the pipe welding apparatus on a first pipe section;

[0020]FIG. 3 is a partially exploded view of the pipe welding apparatus,similar to that shown in FIG. 2;

[0021]FIG. 4 is an elevational view of the pipe welding apparatus ofFIG. 1 taken in the direction of arrow “4”, showing a welding head in anoperational position;

[0022]FIG. 5 is a view of the pipe welding apparatus, similar to thatshown in FIG. 4, showing the welding head in a retracted,non-operational position;

[0023]FIG. 6 is a partially exploded view of the pipe welding apparatus,similar to that shown in FIG. 4;

[0024]FIG. 7 is an elevational view of a pipe welding apparatusaccording to a second embodiment of the invention, showing dual weldingassemblies supported on a main welding carriage, a first welding head inan operational position and a second welding head in an operationalposition;

[0025]FIG. 8 is a view of the pipe welding apparatus, similar to thatshown in FIG. 7, showing the first welding head in a retracted,non-operational position and the second welding head in the operationalposition;

[0026]FIG. 9 is a perspective view of a pipe welding apparatus accordingto a third embodiment of the invention, showing a single weldingassembly supported on a main welding carriage;

[0027]FIG. 10 is an elevational view of the pipe welding apparatus ofFIG. 9 taken in the direction of arrow “10”, showing a typicalinstallation of the pipe welding apparatus on a first pipe section;

[0028]FIG. 11 is a partially exploded view of the pipe weldingapparatus, similar to that shown in FIG. 9;

[0029]FIG. 12 is an elevational view of the pipe welding apparatus ofFIG. 9 taken in the direction of arrow “12”, showing a welding head inan operational position;

[0030]FIG. 13 is a view of the pipe welding apparatus, similar to thatshown in FIG. 12, showing the welding head in a retracted,non-operational position;

[0031]FIG. 14 is a partially exploded view of the pipe weldingapparatus, similar to that shown in FIG. 12;

[0032]FIG. 15 is an enlarged perspective view of the welding head of thethird embodiment taken in isolation (wherein the head is mounted on aframe, a portion of the frame having been removed to show a gear trainhoused therewithin for pivotally oscillating the welding head);

[0033]FIG. 16 is an elevational view of a pipe welding apparatusaccording to a fourth embodiment of the invention, showing dual weldingassemblies supported on a main welding carriage;

[0034]FIG. 17 is another view of the pipe welding apparatus, similar tothat shown in FIG. 16, showing the welding assemblies supported on amain welding carriage; and

[0035]FIG. 18 is a system block diagram of a control system forcontrolling any of the aforementioned welding assemblies.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

[0036] The description which follows, and the embodiments describedtherein, are provided by way of illustration of an example or examplesof particular embodiments which reflect the principles of the presentinvention. These examples are provided for the purposes of explanation,and not of limitation, of those principles and of the invention. In thedescription which follows, like parts are marked throughout thespecification and the drawings with the same respective referencenumerals.

[0037] Referring to FIGS. 1 through 6, in a first embodiment of theinvention, a pipe welding apparatus, generally indicated as 20, isoperable to weld a first pipe section 22 to an adjoining second pipesection 24 along an annular weld joint, groove or seam 26 (seen best inFIG. 2) using the Gas Metal Arc Welding (GMAW) process. The pipe weldingapparatus 20 comprises a motor-driven, main welding carriage 28 and awelding assembly 30 carried thereon.

[0038] In a typical installation of the pipe welding apparatus 20, themain carriage 28 is mounted on an annular band, track or guide 32 whichis supported about the first pipe section 22. The guide 32 extends aboutthe outer circumference of the first pipe section 22 and serves as arail to direct and constrain the circumferential travel of the maincarriage 28 about the first pipe section 22. The main carriage 28 has adriving pinion (not shown) which meshes with a toothed rack (not shown)of the guide 32, thereby providing a means for guiding the main carriage28 around the first pipe section 22. The travel speed of the maincarriage 28 is electronically controlled and travel along the track 32is permissible in both clockwise and counter-clockwise directionsthereby obviating the need for dedicated left-hand and right-handmachines. The pipe welding apparatus 20 may include a sensor (not shown)mounted thereto for determining the circumferential position of thewelding assembly 30 on the first pipe section 22.

[0039] The main carriage 28 has a generally arcuately shaped base 34(seen best in FIG. 1) for facilitating travel about the first pipesection 22. A frame 36 (seen best in FIG. 2) is mounted to the base 34for supporting the welding assembly 30. The frame 36 is comprised of afirst plate 38 disposed generally parallel to the base 34 and a secondplate 40 fixed along one side of the base 34 so as to carry the firstplate 38 clear of the base 34. The welding assembly 30 is mounted on thefirst plate 38.

[0040] The welding assembly 30 comprises a welding head 42 and anelectronically controlled, multi-axis drive mechanism 44 for moving thewelding head 42 relative to the weld seam 26. The multi-axis drivemechanism 44 comprises:

[0041] (a) a horizontal transport assembly 46 for linearly translatingthe welding head 42 in a direction transverse of the weld seam 26;

[0042] (b) a vertical transport assembly 48 for displacing the weldinghead 42 perpendicular to the first pipe section 22 so as to adjust theproximity of the welding head 42 to the weld seam 26; and

[0043] (c) an angular transport assembly 50 for pivotally oscillatingthe welding head 42, in a pendulum style motion, transverse of the weldseam 26.

[0044] Each transport assembly may be operated independently of theother. The welding assembly 30 also has a manually-operated retractingmechanism 52 for pivoting the welding head 42 away from the weld seam26. Each of these mechanisms will now be described in greater detail.

[0045] Referring particularly to FIGS. 2 and 3, the horizontal transportassembly 46 has a motor-driven carriage 54 which is movable relative tothe main carriage 28 in a direction that is generally transverse of theweld seam 26. The carriage 54 has a pair of opposed mounting blocks 56and 58 which support a plate 60 extending therebetween. The mountingblocks 56 and 58 fixedly retain a cylindrical guide member or rod 62that is disposed generally parallel to the plate 60. However, the guiderod 62 is slidably mounted to a pair of mounting blocks 64 and 66 thatare anchored to the first plate 38 of frame 36 thereby securing thehorizontal transport assembly 46 to the main carriage 28. Each mountingblock 64, 66 has a bore 68 defined therein located to receive the guiderod 62 therethrough. The bore 68 has a bushing 70 and bearings 72mounted thereabout to permit the guide rod 62 to slide within themounting blocks 64 and 66 when the horizontal transport assembly 46 isactuated.

[0046] An electronically controlled motor 74 drives the carriage 54relative to the main carriage 28. The motor 74 is mounted to a structure76 that is fixedly connected to the mounting block 66. The motor 74 hasan output shaft onto which a cog 80 is mounted. The cog 80 hasprojections (not shown) such as ribs which engage correspondingprojections (not shown) on the underside of a band or belt 82. The belt82 is connected to a second cog 84 which is mounted to a threaded shaft86 that is fitted through the structure 76. The belt 82, the second cog84 and the threaded shaft 86 co-operate to form a cog pulley 88. Throughthis arrangement the threaded shaft 86 is precluded from translationalmovement relative to the main carriage 28 but is permitted rotationalmotion. A threaded nut 90 which is fixedly connected to the guide rod 62and the mounting block 58 is adapted to receive, and threadingly engage,the shaft 86, thereby forming a recirculating ball-screw mechanism,known to those skilled in the art. As will be explained in greaterdetail below, this mechanism when actuated will urge the carriage 54 totranslate in a direction transverse to the weld seam 26.

[0047] Actuating the motor 74 causes the belt-driven shaft 86 to rotate.As the shaft 86 rotates, its threading engages the counterpart threadingof the nut 90. Since the shaft 86 is constrained to only a rotationalmotion, the nut 90 is urged to travel along shaft 86. Given that the nut90 is fixed to the guide rod 62, as the nut 90 travels along the shaft86 the guide rod 62 is urged to slide within the bore 68 of the mountingblocks 64 and 66, thereby causing the carriage 54 to translate in adirection transverse of the weld seam 26.

[0048] As the horizontal transport assembly 46 controls the lineartranslation of the welding head 42 it can thus impart an oscillatoryside-to-side movement or shuttle-type motion to the welding head 42transverse of the welding seam 26. As will be appreciated by thoseskilled in the art, this type of oscillatory motion is particularlyuseful when welding pipe sections that have relatively large angledbevels. In such cases, the welding head 42 can be moved to effectivelybridge the gap which results from a wider welding seam, thereby reducingthe number of weld passes required to achieve a strong weld.

[0049] Referring particularly to FIGS. 2 and 3, the vertical transportassembly 48 is supported on the carriage 54 of the horizontal transportassembly 46. The assembly 48 comprises a structural frame 100 and amotor-driven carriage block 102 which imparts vertical motion to thewelding head 42 relative to the first pipe section 22. The structuralframe 100 comprises a plate 104 which is generally parallel to the plate60 of the carriage 54 and a transverse member 106 for supporting plate104. A cylindrical guide member or rod 108 and a threaded shaft 110 aremounted so as to extend between plates 60 and 104 of the horizontal andvertical transport assemblies 46 and 48, respectively.

[0050] The carriage block 102 has a first bore 112 defined thereinlocated to receive guide rod 108 therethrough. The bore 112 has abushing 114 and bearings 116 mounted thereabout to permit the carriageblock 102 to slide along the guide rod 108 when the vertical transportassembly 48 is actuated. The carriage block 102 also has a secondthreaded bore 118 which is adapted to receive and threadingly engage theshaft 110. As will be explained in greater detail below, the guide rod108 and the shaft 110 co-operate to translate the carriage block 102relative to the main carriage 28 in a direction that is perpendicular tofirst pipe section 22.

[0051] An electronically-controlled motor 120 is secured to thestructural frame 100. The motor 120 has an output shaft 122 whichextends through the plate 104 and is connected to a cog 124. Projections(not shown) such as ribs on the cog 124 engage corresponding projections(not shown) on the underside of a band or belt 126 which is connected toa second cog 128 that, in turn, is mounted to the threaded shaft 110.Belt 126, second cog 128 and threaded shaft 110 co-operate to form a cogpulley 130. Through this arrangement, the threaded shaft 110 isprecluded from translational movement in a direction perpendicular tothe first pipe section 22, but is permitted to rotate in place.

[0052] Actuating the motor 120 causes the belt-driven threaded shaft 110to rotate. As the shaft 110 rotates, its threading engages thecounterpart threading of second bore 118. Since the threaded shaft 110is constrained to only a rotational motion, the carriage block 102 isurged to travel along the threaded shaft 110 and slide along the guiderod 108. The welding head 42 is thus urged to move in a directiongenerally perpendicular to first pipe section 22. In this manner, thevertical transport assembly 48 can be used to control the verticalposition of the welding head 42 relative to the weld seam 26. Thecontact-to-work offset of the tip of the welding head 42 relative to theweld seam 26 may thus be adjusted as required for the welding process.

[0053] Referring particularly to FIGS. 2 and 6, the angular transportassembly 50 comprises a frame 140 which is pivotally mounted on thecarriage block 102 of the vertical transport assembly 48. Anelectronically-controlled motor 142 is mounted to the frame 140 fordriving the pivotal oscillation of the welding head 42 about a pivot144. More specifically, the motor 142 drives a cam 146 eccentricallymounted on the motor relative to its drive shaft (not shown). The cam146 engages a cam follower 148 which comprises a yoke 150 having a pairof arms 152 and 154. A mount or bracket 156 is fixed to the yoke 150 andcarries the welding head 42. The yoke 150 also has a shaft or pin 158which extends perpendicular to the yoke 150 to be received in a bore 160defined in the frame 140. The bore 160 of the frame 140 has a bushing162 and bearings 164 mounted thereabout to permit the rotation of thepin 158 therewithin. As will be understood by those skilled in the art,the foregoing arrangement forms the pivot 144 about which the weldinghead 42 will oscillate when the angular transport assembly 50 isactuated.

[0054] Actuating the motor 142 causes the cam 146 to rotate. Thisrotational motion is transferred to yoke 150 through its arms 152 and154. As the yoke 150 follows the movement of the cam 146, the pin 158 isurged to rotate within the bore 160, causing the yoke 150 to adopt anangular motion relative to the pivot 144. This causes the welding head42 to travel along an arcuate path in a direction generally transverseof weld seam 26.

[0055] During a typical operation of the pipe welding apparatus 20, theangular transport assembly 50 may be actuated to urge the welding head42 to pivotally oscillate transversely of the weld seam 26 with apendulum-like motion. As will be appreciated by those skilled in theart, this type of oscillatory movement may be particularly useful whenwelding thick, heavy pipes having a deep, narrow weld bevel, as itallows the arc generated by the welding head 42 to reach the bottom ofthe weld seam 26. This tends to create better penetration in the weldarea and discourage the occurrence of non-fusion defects at the bottomof the weld seam 26. Consequently, fewer repairs may be required duringthe welding process.

[0056] The rate and velocity profile (speed and/or direction of motionat any point during an oscillation period) parameters of the oscillatorymotion of the welding head 42 may be electronically adjusted to suit aparticular welding situation. The oscillation width or radial angle maybe varied incrementally during the welding operation. In addition, theangular transport assembly 50 may also have oscillation dwellcapabilities which would allow the welding head 42 to dwell or pause ina pre-selected position in its arcuate path for a pre-determined periodof time.

[0057] The pipe welding apparatus 20 may also be provided with a weldseam tracking system (not shown). This system could be operable to sensethe position of the welding head 42 relative to the weld seam 26 and tomaintain oscillation of the welding head 42 centered in the weld seam26.

[0058] During a typical operation of the pipe welding apparatus 20, thehorizontal and angular transport assemblies 46 and 50, respectively, maybe actuated individually, sequentially, or simultaneously, i.e., incombination one with the other. The pipe welding apparatus 20 is thuseasily adaptable to different welding situations and capable of handlinga variety of weld preparation groove geometries. For instance, where theweld seam is wide, shuttle-type oscillation may be used to ensure thatthe weld gap is properly bridged. Alternatively, the pipe weldingapparatus can accommodate a narrow weld seam by switching to pivotaloscillation to maximize tip penetration in the weld seam. The combineduse of the transport assemblies 46 and 50 can provide added versatilityto the automated welding process and may permit the pipe weldingapparatus 20 to more closely simulate the motion profile of amanually-controlled and operated welding head. Simultaneous orsynchronized actuation of the transport assemblies 46 and 50 maygenerate more complex welding head motion profiles as the linear,shuttle-type motion is combined with the pivotal oscillating motion.Notably, shuttle-type oscillation can be used to horizontally displacethe pivot point about which the welding head 42 is oscillated. Theextent to which motion of the welding head 42 can be controlled alsoenables a single pipe welding apparatus 20 to perform multi-stagewelding procedures which have conventionally been performed by severaldifferent welding machines (e.g., dedicated “root-pass”, “fill-pass” and“cap-pass” machines).

[0059] Likewise, the vertical and angular transport assemblies 48 and 50can be actuated simultaneously to vertically displace the pivot pointabout which the welding head 42 oscillates. In this way, the arcuatepath or trajectory of the welding head 42 can be modified or altered forincreased versatility. Raising the pivot point relative to the firstpipe section 22 during oscillation will produce a more pronouncedcurving trajectory for the welding head 42. Conversely, lowering thepivot point relative to the first pipe section 22 during oscillationwill tend to flatten out the trajectory of the welding head 42.

[0060] It is also possible to actuate the three transport mechanisms 46,48 and 50 in combination one with the other. For example, the horizontaland the vertical transport mechanisms 46 and 48, respectively can beactuated to displace the pivot point while the angular transportmechanism 50 operates to oscillate the welding head 42.

[0061] The retracting mechanism 52 will now be described with referenceto FIGS. 3, 4 and 5. As previously indicated, the frame 140 of theangular transport assembly 50 is pivotally mounted on the carriage block102 of the vertical transport assembly 48. The frame 140 has a bore 170defined therein located to receive a mounting shaft or pin 172 extendingfrom the carriage block 102. The bore 170 has a bushing 174 and bearings176 mounted thereabout to permit the frame 140 (and welding head 42) tobe rotated approximately 90° degrees about the mounting pin 172.Accordingly, the welding head 42 may be moved from an operationalposition (shown on FIG. 4) to a retracted, non-operational position(shown on FIG. 5). In the operational position, the welding head 42 isdisposed generally perpendicular to the first pipe section 22 to effectthe welding process along the weld seam 26. In the operational position,the welding head 42 is directed away from the first pipe section 22 in adirection roughly parallel to the weld seam 26. While in this position,the welding head 42 may be readily accessible for the purpose ofcleaning, servicing or inspection. A clamp 182 mounted to the frame 140and the carriage block 102 is manually operable to lock the welding head42 in the operational or retracted positions.

[0062] To actuate the retracting mechanism 52, the clamp 182 is firstreleased. The frame 140 is then urged to rotate about the mounting pin172, thereby causing the welding head 42 to be moved from the operatingposition to the retracted position. The clamp 182 is then engaged tosecure the welding head 42 in the retracted position while cleaning orservicing activities are being carried out. Upon completion of theseactivities, the clamp 182 is released to permit the welding head 42 tobe returned to the operational position. When returned to theoperational position, the welding head 42 is in the precise position itoccupied prior to retraction. Actuation of the retracting mechanism 52does not alter the pre-adjusted contact-to-work offset of the tip of thewelding head 42 relative to the weld seam 26. Accordingly, no furtherpositional adjustment of the welding head 42 is necessary to allow thewelding operation to resume as before.

[0063] The welding head 30 is now described in greater detail. Thewelding head 30 comprises a torch 190 for applying consumable weld metaland heat to the weld seam 26. In accordance with the GMAW process, thetorch 190 uses welding wire as a consumable electrode to form an arcbetween the power source and the weld seam 26. As the weld bead forms,the wire contributes molten metal to the weld puddle. Alternatively, thetorch 190 could be designed to perform the Flux Core Arc Welding process(FCAW) or the Peak Arc Welding process (PAW).

[0064] In the GMAW process, welding wire is constantly driven throughthe torch 190 by a motorized wire feed device (not shown) supplied bywire spools. The wire spools (not shown) are not mounted to the maincarriage 28 and as a result, the use of small and very costly weldingwire spools is avoided. Larger off-the-shelf wire spools may be usedinstead, leading to reduced welding wire costs. Moreover, since the wirespools are not carried on the main carriage 28, the weight of the pipewelding apparatus 20 may be considerably lighter than conventionalwelding apparatus. If desired, current sensor (not shown) may beemployed for detecting welding amperage in order to ensure that the wirefeed device delivers sufficient wire to maintain the proper wire stick.

[0065] As previously indicated, the horizontal, and the angulartransport assemblies 46 and 50, respectively, may be actuatedindividually or in combination one with the other to encourageappropriate distribution of the consumable electrode over the weld seam26.

[0066] Although, in the first embodiment, only a single welding assembly30 is carried on the main carriage 28 of the pipe welding apparatus 20,the welding assembly 30 is sufficiently compact and self contained thatmultiple welding assemblies may be carried on the main carriage 28.FIGS. 7 and 8 show a second embodiment of the invention in which a pipewelding apparatus, generally indicated as 200, has a first weldingassembly 202 and a second welding assembly 204 supported on a maincarriage 206. The main carriage 206 is similar in construction andoperation to main carriage 28. Attachment of each welding assembly 202,204 to main carriage 206 is achieved in a manner similar to that ofwelding assembly 30 to main carriage 28.

[0067] Each welding assembly 202, 204 is similar in construction andoperation to the welding assembly 30. Each welding assembly 202, 204 hasa welding head 208, 210 and an electronically controlled mechanism 212,214 for controlling the movement of welding head 208, 210 relative toweld seam 26. Mechanisms 212 and 214 are similar to the multi-axismechanism 44. Each welding assembly 202, 204 also has amanually-operated retracting mechanism 228, 230 for pivoting the weldinghead 208, 210 away from the weld seam 26.

[0068] The welding assembly 202 is disposed on the main carriage 206opposite the welding assembly 204 in such a fashion that one is themirror image of the other. This arrangement permits the rotationalretraction of one welding head without physically interfering with theother (see FIG. 8). When the retracting mechanism 228 is actuated, thewelding head 208 of the first welding assembly 202 will be rotated in aclockwise direction from an operational position (shown on FIG. 7) to aretracted, non-operational position (shown on FIG. 8). When theretracting mechanism 230 is actuated, the welding head 210 of the secondwelding assembly 204 will be subject to counter-clockwise rotation froman operational position to a retracted, non-operational position (notshown). In this way, the welding heads 208 and 210 may be rotated awayfrom the first pipe wall 22 and each other for rapid and easy servicing.

[0069] Each welding assembly 202, 204 operates independently of theother, except in regard to the overall travel speed of the main carriage206. This particular welding parameter is necessarily common to bothwelding assemblies 202 and 204. The operational independence of weldingassemblies 202 and 204 provides a high level of versatility that may behighly beneficial in the field use of automated welding procedures.Different oscillation schemes may be generated for each welding head208, 210. In addition, the contact-to-work offsets of each welding head208, 210 may be varied for added versatility, as shown in FIG. 7,thereby allowing the two weld processes to take place at differentelevations.

[0070] In the first and second embodiments, the multi-axis drivemechanisms 44 and 212 and 214, respectively, are constructed such thatthe carriage of the horizontal transport assembly provides a platformfor supporting the vertical and angular transport assemblies. However,alternative arrangements of the transport assemblies are possible. Forinstance, the hierarchy of horizontal and vertical transport assembliescan be reversed such that the horizontal and angular transportassemblies may be carried on the carriage of the vertical transportassembly. FIGS. 9 through 15 show a third embodiment in which a pipewelding apparatus, generally indicated as 250, comprises a motor-driven,main welding carriage 252 and a welding assembly 254 carried thereon.The pipe welding apparatus 250 is installed in generally the same mannerabout the first pipe 22 as the pipe welding apparatus 20.

[0071] The main carriage 252 has a generally arcuately shaped base 256similar to the base 34 of pipe welding assembly 20. A frame 258 (seenbest in FIG. 10) is mounted to the base 256 for supporting the weldingassembly 254. The frame 258 is comprised of a vertical wall portion 260extending in a generally perpendicular direction away from the base 256and a horizontal support plate 262 integrally formed with and extendingtransversely of the vertical wall portion 260. The welding assembly 254is carried by the frame 258.

[0072] The welding assembly 254 comprises a welding head 270 similar inconstruction and operation to the welding head 42 of the pipe weldingapparatus 20, and an electronically controlled, multi-axis drivemechanism 272 for moving the welding head 270 relative to the weld seam26. The multi-axis drive mechanism 272 comprises:

[0073] (a) a horizontal transport assembly 274 for linearly translatingthe welding head 270 in a direction transverse of the weld seam 26;

[0074] (b) a vertical transport assembly 276 for displacing the weldinghead 270 perpendicular to the first pipe section 22 so as to adjust theproximity of the welding head 270 to the weld seam 26; and

[0075] (c) an angular transport assembly 278 for pivotally oscillatingthe welding head 270, in a pendulum style motion, transverse of the weldseam 26.

[0076] The welding assembly 254 also has a manually-operated retractingmechanism 280 for pivoting the welding head 270 away from the weld seam26. Each of these mechanisms will now be described in greater detail.

[0077] Referring particularly to FIGS. 10, 11 and 14, the verticaltransport assembly 276 has a motor-driven carriage 282 which impartsvertical motion to the welding head 270 relative to the first pipesection 22. The carriage 282 has a pair of opposed mounting blocks 284and 286 which support a vertical plate 288 extending therebetween. Themounting blocks 284 and 286 fixedly retain a pair of cylindrical guidemembers or rods 290 and 292 that are disposed generally parallel to thevertical plate 288. However, the guide rods 290 and 292 are slidablymounted to a pair of mounting blocks 294 and 296 that are integrallyformed with frame 258 thereby securing the vertical transport assembly276 to the main carriage 252. Each mounting block 294, 296 has a pair ofbores generally indicated as 298 defined therein located to receive theguide rods 290 and 292 therethrough. Each bore 298 has a bushing 300 andbearings 302 mounted thereabout to permit the guide rod 290 or 292, asthe case may be, to slide within the mounting blocks 294 and 296 whenthe vertical transport assembly 276 is actuated.

[0078] An electronically controlled motor 304 mounted to the horizontalsupport plate 262 drives the carriage 282 relative to the main carriage252. The motor 304 has an output shaft onto which a cog 306 is mounted.The cog 306 has projections (not shown) such as ribs which engagecorresponding projections (not shown) on the underside of a band or belt308. The belt 308 is connected to a second cog 310 which is mounted to athreaded shaft 312 that is fitted through the horizontal support plate262. The belt 308, the second cog 310 and the threaded shaft 312co-operate to form a cog pulley 314 similar to cog pulley 130 of pipewelding apparatus 20. Through this arrangement, the threaded shaft 312is precluded from translational movement relative to main carriage 252,but is permitted to rotate in place.

[0079] The mounting block 284 has a threaded bore 316 that is adapted toreceive and threadingly engage the shaft 312. The guide rods 290 and 292and the shaft 312 co-operate to translate the carriage 282 relative tothe main carriage 252 in a direction that is perpendicular to first pipesection 22.

[0080] Actuating the motor 304 causes the belt-driven shaft 312 torotate. As the shaft 312 rotates, its threading engages the counterpartthreading of the bore 316. Since the shaft 312 is constrained to only arotational motion, the carriage 282 is urged to travel along thethreaded shaft 312 and slide along the guide rods 290 and 292. Thewelding head 270 is thus urged to move in a direction generallyperpendicular to first pipe section 22. The extent to which the weldinghead 270 may be displaced vertically depends on the size of an apertureor slot 318 (as best shown in FIG. 14) defined within vertical wallportion 260. As will be explained below in greater detail, a mountingrod, pin or shaft 319 is mounted to extend through slot 318 to connectthe horizontal transport assembly 274 to the angular transport assembly278. As a result, the slot 318 constrains the vertical displacement ofwelding head 270 to a pre-selected range.

[0081] Referring particularly to FIGS. 10 and 11, the horizontaltransport assembly 274 is supported on the carriage 282 of the verticaltransport assembly 276. The assembly 274 comprises a structural frame320 and a motor-driven carriage block 322 which is movable relative tothe main carriage 252 in a direction that is generally transverse of theweld seam 26. The structural frame 320 comprises a horizontal plate 324which is mounted to vertical plate 288 of carriage 282 and a verticalplate 326 connected to the horizontal plate 324. A cylindrical guidemember or rod 328 and a threaded shaft 330 are mounted so as to extendbetween vertical plates 288 and 326.

[0082] The carriage block 322 has a first bore 332 defined thereinlocated to receive guide rod 328 therethrough. The bore 332 has abushing 334 and bearings 336 mounted thereabout to permit the carriageblock 322 to slide along the guide rod 328 when the horizontal transportassembly 274 is actuated. The carriage block 322 also has a secondthreaded bore 338 that is adapted to receive and threadingly engage theshaft 330. As will be explained in greater detail below, the guide rod328 and the shaft 330 co-operate to translate the carriage block 322relative to the main carriage 252 in a direction that is generallytransverse of the weld seam 26.

[0083] An electronically-controlled motor 340 is secured to thestructural frame 320. The motor 340 has an output shaft 342 whichextends through the vertical plate 326 and is connected to a cog 344.Projections (not shown) such as ribs on the cog 344 engage correspondingprojections (not shown) on the underside of a band or belt 346 which isconnected to a second cog 348 that, in turn, is mounted to the threadedshaft 330. Belt 346, second cog 348 and threaded shaft 330 co-operate toform a cog pulley 350 similar to the cog pulley 88 of pipe weldingapparatus 20. Through this arrangement, the threaded shaft 330 isprecluded from translational movement relative to the main carriage 252but is permitted rotational motion.

[0084] Actuating the motor 340 causes the belt-driven threaded shaft 330to rotate. As the shaft 330 rotates, its threading engages thecounterpart threading of second bore 338. Since the threaded shaft 330is constrained to only a rotational motion, the carriage block 322 isurged to travel along the threaded shaft 330 and slide along the guiderod 328. The welding head 270 is thus urged to move in a directiontransverse of the weld seam 26.

[0085] Referring particularly to FIGS. 10, 11 and 15, the angulartransport assembly 278 comprises a frame 360 which is pivotally mountedto the carriage block 322 of the horizontal transport assembly 274. Anelectronically-controlled motor 362 is mounted to the frame 360 to drivea gear train 364 housed within the frame 360. The gear train 364comprises a first worm gear 366, a second gear, such as a pinion spurgear 368 and a third gear, such as a crown spur gear 370. The worm gear366 has a vertical worm 372 driven by the motor 362 which works upon atoothed wheel 374. The toothed wheel 374 is mounted on a firsthorizontal shaft 376 which is pivotally mounted to the frame 360. Thetoothed wheel 374 is coupled to the pinion spur gear 368 such thatrotation of the wheel 374 causes a corresponding rotation of the gear368. The crown spur gear 370 is mounted to a second horizontal shaft 378and disposed within the gear train 364 for engagement by pinion spurgear 368. A mount or bracket 380 is securely fixed to the secondhorizontal shaft 378 and carries the welding head 270.

[0086] As will be understood from the foregoing, the motor 362 drivesthe worm gear 366 causing the pinion spur gear 368 to rotate about thefirst horizontal shaft 376. This rotational motion is transferred to thecrown spur gear 370 and the second horizontal shaft 378 is urged torotate. Since the welding head 270 is fixedly secured to the shaft 278,as the shaft 278 rotates the welding head 270 is pivoted transversely ofthe weld seam 26. Pivotal oscillation of the welding head 270 isachieved by alternating the driving direction of the motor (i.e.clockwise to counter-clockwise). In this manner, a direct drive systemis provided, however, those skilled in the art will understand thatother gear train configurations are possible and may be employed togenerate the pivotal oscillation.

[0087] As with the angular transport assembly 50 of the pipe weldingapparatus 20, the rate, magnitude and velocity profile parameters of theoscillatory motion of the welding head 270 may be adjustedelectronically. Similarly, the angular transport assembly 278 may alsohave oscillation dwell capabilities. Likewise, during a typicaloperation of the pipe welding apparatus 250, the horizontal and angulartransport assemblies 274 and 278, respectively, may be actuatedindividually, sequentially, or simultaneously, i.e., in combination onewith the other.

[0088] The retracting mechanism 280 will now be described with referenceto FIGS. 11, 12 and 13. As previously mentioned, the frame 360 of theangular transport assembly 278 is pivotally mounted to the carriageblock 322 of the horizontal transport assembly 274. The mounting shaft319 extends from the carriage block 322 through the slot 318 of thevertical wall portion 260 and is received within a bore 390 defined inthe frame 360. The bore 390 has a bushing 392 and bearings 394 mountedthereabout to permit the frame 360 (and welding head 270) to be rotatedapproximately 90° degrees in a clockwise direction about the mountingshaft 319. Accordingly, the welding head 270 may be moved from anoperational position (shown on FIG. 12) to a retracted, non-operationalposition (shown on FIG. 13). Rotation of the welding head 270 about themounting shaft 319 is constrained by a pin and slot arrangement 400.Arrangement 400 has a pin 402 connected to mounting shaft 319 which ismounted to travel within an arcuate slot 404 defined within the frame360.

[0089] In the first position 396, the welding head 270 is disposedgenerally perpendicular to the first pipe section 22 to effect thewelding process along the weld seam 26. In the retracted position, thewelding head 270 is directed away from the first pipe section 22 in adirection roughly parallel to the weld seam 26. While in this position,the welding head 270 may be readily accessible for the purpose ofcleaning, servicing or inspection. A clamp 406 having a threaded cap408, and mounted to the frame 360 and the vertical wall member 260, ismanually operable to lock the welding head 270 in the operationalpositions.

[0090] To actuate the retracting mechanism 280, the clamp 406 is firstreleased by unscrewing the cap 408. The frame 360 is then urged into aclockwise rotation about the mounting shaft 319, thereby causing thewelding head 270 to be moved from the first position 396 to the secondposition 398. The cap 408 is then tightly screwed to secure the weldinghead 270 in the second position 398 while cleaning or servicingactivities are being carried out. Upon completion of these activities,the clamp 406 is released to permit the welding head 270 to be returnedto the first position 396.

[0091] In like fashion to welding assembly 30 of pipe welding apparatus20, welding assembly 254 is sufficiently compact and self contained suchthat multiple welding assemblies may be carried on the main carriage252. FIGS. 16 and 17 show a fourth embodiment of the invention in whicha pipe welding apparatus, generally indicated as 420, has a firstwelding assembly 422 and a second welding assembly 424 supported on amain carriage 426. The arrangement of assemblies 422 and 424 on the maincarriage 426 is similar to that of assemblies 202 and 204 on the maincarriage 206 of pipe welding apparatus 200. The construction andoperation of welding assemblies 422 and 424 resemble that of weldingassembly 254 of pipe welding apparatus 250.

[0092] Various pipe welding apparatus 20, 200, 250 and 420 have beendescribed as having electronically-controlled transport mechanisms formoving the welding head relative to the weld seam 26.

[0093] A preferred control system 430 is shown in block diagram form inFIG. 18. The system comprises a memory 432 in which a plurality ofprogrammable profiles are stored. Each such profile describes a twodimensional or x-y trajectory for the tip of the welding torch, theframe of reference being situated such that the x axis represents adirection transverse to the weld seam and the y-axis represents adirection vertical to the weld seam. The trajectory also specifies theorientation or angle of the welding torch tip relative to the y-axis. Ifdesired, a graphical user interface may be employed in conjunction withan external computer to describe and plot the foregoing three parametersover time, including oscillation characteristics such as dwell time, aswill be known to those skilled in this art. The plotted data can thus bedownloaded into the memory 432 via a communications link (not shown) toform a particular trajectory. In addition, each profile also includeswelding parameters such as desired current, voltage, main carriagetravel speed and wire feed speed.

[0094] A block 436 of the control system 430 selects a particularprofile stored in the memory 432 in order to control the weldingapparatus at any given point in time. The selected profile is preferablybased on the circumferential position of welding apparatus about thepipe sections. For this reason the control system 430 includes acircumferential position sensing subsystem 434, as known in the art perse, which feeds circumferential position data to the selector block 436.In this manner weld profiles can be automatically selected for differentcircumferential passes of the welding apparatus, as well as fordifferent angular sectors, as may be needed, for instance, to control“drip” of any molten weld metal due to the influence of gravity as thewelding apparatus travels around the pipe sections. The selectedtrajectory and welding parameters are fed into a motor controller block438 which controls the motors 440, 442 and 444 of the horizontal,vertical and angular transport mechanisms, respectively. The controller438 preferably employs a closed loop based control law such as wellknown p.i., p.i.d, or state-space based error feedback loops. Based onone or more such loops the controller 438 provides commands to currentamplifiers (not shown) which supply the desired currents to the motors440, 442 and 444. Each motor 440, 442, 444 has a feedback sensor fordetermining its position. This information is fed back to the controller438 in order to enable it to compute the absolute position and angle ofthe welding tip and thereby to control the motors 440, 442 and 444 so asto minimize any error between the desired and actual position.

[0095] Sensors 446 and 448 provide the controller 438 with feedback asto the actual or measured welding current (I) and welding voltage (V).The controller 438 uses this feedback to calculate the instantaneousheat input into the weld, which is measured as (V)(I)*/60(S), where S isthe travel speed of the main carriage. In preferred embodiments thecontroller 438 employs an additional control loop to maintain the heatinput to the desired amount by varying the vertical distance between thewelding tip and the welding seam, thereby varying the welding currentand welding voltage characteristics. This control loop overrides thecontrol loop for the vertical component of the trajectory (i.e., they-axis) and thus the vertical component or parameter of the trajectorystored in the memory 432 can be considered to be a nominal or baselineparameter only which will be varied in response to actual weldingconditions. In this manner, the control system 430 ensures that minimumheat input is supplied to the weldment in accordance withspecifications.

[0096] It will be understood by those skilled in the art that theforegoing description is made with reference to illustrative embodimentsof the invention and that other embodiments employing the principles ofthe invention may be envisaged.

We claim:
 1. A movable welding apparatus for forming a weld pass, saidapparatus comprising: a carriage for movement along a track mounted to asurface of an object to be welded, the carriage having a direction ofmovement lengthwise relative to the weld pass to be formed; a weldinghead mounted to said carriage; said welding head having a first degreeof freedom permitting cross-wide motion thereof relative to thedirection of motion of the carriage; said welding head having a seconddegree of freedom permitting rocking motion of said welding head; saidapparatus being operable to vary position of said welding head in eachof said first and second degrees of freedom while said welding head isin operation forming the weld pass.
 2. The moveable welding apparatus ofclaim 1 wherein said apparatus is movable in both said first and seconddegrees of freedom simultaneously.
 3. The movable welding apparatus ofclaim 1, wherein said first degree of freedom is a transverse lineardegree of freedom.
 4. The movable welding apparatus of claim 1, whereinsaid second degree of freedom is a pivoting degree of freedom.
 5. Themovable welding apparatus of claim 4, wherein said second degree offreedom is a pivoting degree of freedom permitting cross-wideoscillation of said welding head relative to the direction of movementof the carriage.
 6. The movable welding apparatus of claim 1 whereinsaid first degree of freedom is a transverse linear degree of freedom,said second degree of freedom is a transverse pivoting degree offreedom, and said welding head is movable simultaneously with respect toboth said first and second degrees of freedom to produce a complexcross-wide rocking motion of said welding head relative to the directionof travel of said carriage.
 7. The movable welding apparatus of claim 1wherein said welding head has a third degree of freedom permittingvariation of spacing of said welding head normal to the surface.
 8. Themovable welding apparatus of claim 1 further comprising a controlleroperable to actuate the first and the second transport mechanismssimultaneously to change a vertical position of a pivoting point of saidwelding head in accordance with a pre-determined trajectory.
 9. Themovable welding apparatus of claim 1 further comprising feedbackapparatus mounted to monitor operating parameters of said weldingapparatus during operation thereof, and control apparatus, said controlapparatus being operable in response to signals from said feedbackapparatus to vary position of said welding head during operationthereof.
 10. A welding apparatus comprising: a carriage for movementalong a track adjacent to a location for forming a weld pass; a weldingtorch movably mounted to said carriage; a first transport mechanismoperable to move said torch transversely relative to the movement ofsaid carriage; a second transport mechanism operable angularly to movesaid torch relative to the movement of said carriage; and said first andsecond transport mechanisms being operable to move said torch relativeto said carriage during operation of said torch to form said weld pass.11. The welding apparatus of claim 10 wherein said first and secondtransport mechanisms are simultaneously operable during operation ofsaid torch.
 12. The welding apparatus of claim 10 wherein said first andsecond transport mechanisms are simultaneously operable to oscillatesaid welding torch during formation of the weld pass.
 13. The weldingapparatus of claim 12 wherein said first and second transport mechanismsare simultaneously operable to oscillate said torch transverselyrelative to movement of said carriage while forming the weld pass. 14.The welding apparatus of claim 10, further comprising the thirdtransport mechanism operable to establish a contact-to-work offset ofsaid torch.
 15. The welding apparatus of claim 14 wherein said thirdtransport mechanism is operable to vary said contact to work offset. 16.The welding apparatus of claim 15 wherein said third transport mechanismis operable while said carriage is in motion.
 17. The welding apparatusof claim 15 wherein operation of said first, second, and third transportmechanisms is electronically controlled.
 18. The welding apparatus ofclaim 15 wherein said first, second and third transport mechanisms aresimultaneously operable.
 19. The welding apparatus of claim 10 whereinsaid second transport mechanism is a pivoting mechanism for pivotingsaid torch about an axis aligned with the direction of movement of saidcarriage.
 20. A welding apparatus for forming weld pass seams betweentwo abutting work pieces, said apparatus comprising: a carriage forfollowing a path along a surface of one of the abutting work pieces; atorch movably mounted to a carriage; a first transport assembly operableto linearly translate said torch transversely relative to the path ofsaid carriage; a second transport assembly operable to pivot said torchabout an axis parallel to the path of said carriage; and, while saidtorch is in operation, said first and second transport assemblies beingsimultaneously operable laterally to oscillate said torch duringformation of a weld pass.